There's a lot that goes into an exchange (outlook for you hehe) environment especially when it comes to "tuning". First you need to look at your storage groups and how big they are. The max per SG you want to see is 18gig. After that you need to start splitting them into bigger groups. You also have to be concerned with RPC traffic, and calls to your DC (holding a global catalog role). You also need to run offline defrags to clean up. Even if you start cleaning out mailboxes, the database stays the same size until it's defragged.
It's not so easy to "tune" but it also takes an IT staff with the balls to do it and a management structure that will let you touch it. Most of the time you'll find that upper management freaks out when you say you want to touch exchange.
We had several similar issues here and yes it was the amount of e-mails people were storing. a 4gig mailbox is retarded. However, we have the balls and the management to say "Buy an archiving tool and we'll get this shit cleaned up".
I have a word limit on my cynicism